Week-7 Challenge: DC Motor Control

OBJECTIVE:

• To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
  • To comment on the armature current shoot-up from the scope results.
  • To refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
• To develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results.
• To explain in brief about the operation of BLDC motor.

SOLUTION:

OBJECTIVE - 1:

SPEED CONTROL OF A DC MOTOR USIGN BJT H-BRIDGE:

• The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.
• The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
• The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

SIMULATION

• The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.
• 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

WORKING

• H Bridge is a simple electronic circuit which enables us to apply voltage to load in either direction. It is commonly used in robotics applications to control DC Motors. By using H Bridge, we can run DC Motor in clockwise or anticlockwise directions. This circuit is also used to produce alternating waveforms in inverters.

When switches S1 and S4 are switched on, the motor runs in clockwise direction.

When S2 and S3 are switched on, the motor runs in an anticlockwise direction.

In this circuit all transistors are wired as switches. An NPN transistor (Q3 and Q4) will be ON when we give HIGH to it and a PNP transistor (Q1 and Q2) will be ON when we give LOW to it. So, when (A = LOW, B = HIGH, C = LOW, D = HIGH), transistors Q1 & Q4 will be ON and Q2 & Q3 will be OFF, so the motor rotates in clockwise direction. Similarly, when (A = HIGH, B = LOW, C = HIGH, D = LOW), transistors Q2 & Q3 will be ON and transistor Q1 & Q4 will be OFF, thus the motor rotates in anticlockwise direction. 1N4148 (D1 ~ D4) is uses as freewheeling diode as it is a fast-switching diode. It avoids problems due to negative voltage produced by the back emf the dc motor. Resistors R1 – R4 are used to limit the input current of transistors and are designed in such a way that transistors will work as a switch. Transistor should be chosen according to the current requirements of DC Motor.

DESIGN OF INPUT RESISTORS:

• I_L = Max. Motor Current
• Ib = I_L/β, Minimum input current required to flow current I_L through the load (active region).
• Ib’ = 10xIb, To make sure that transistor works in saturation region.
• R_in = (Vin – Vbe)/Ib’
• Take Vbe = 0.7V

This circuit is also used in inverters for producing alternate current across the load. In Square wave inverters, square wave is applied to inputs A, B and in sine wave inverters Pulse Width Modulated square wave is applied to inputs A, B. Freewheeling diodes D1 – D4 can be avoided for resistive loads.

SIMULINK MODEL:

RESULTS:

CONCLUSION:

• From the above result scope, it can be clearly observed that when the motor is changing its direction from clockwise rotation to anticlockwise rotation the armature current shoots-up to a higher value as compared to the initial starting condition.
• When the motor starts to operate in the reverse condition there is much higher shoot-up in the armature current because in this condition the motor requires more current and voltage i.e. the load current and the magnetic field current in order to move the motor in the reverse direction or in the anticlockwise direction.
• So, when the motor changes its direction of motion from clockwise to anti-clockwise or from forward to reverse there is a huge shoot-up in the armature current.
• This shoot-up in the current can be managed by adding a resistor and providing resistance to the circuit so that the armature current doesn't shoot-up to such a higher value.

THE FOUR QUADRANT CHOPPER DC DRIVE:

• The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.
• The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

SIMULATION:

• Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).
• Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.
• The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
• The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.
• The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
• At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.
• At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.
• At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

NOTES:

1) The power system has been discretized with a 1 us time step. The speed and current controllers use a 100 us and 20 us sampling time respectively in order to simulate a microcontroller control device.

2) In order to reduce the number of points stored in the scope memory, a decimation factor of 25 is used. Some transitions may thus not appear in scope. To view detailed simulation results, reduce the decimation factor to 1.

3) A simplified version of the model using an average-value converter can be used by selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The time step can then be increased up to the smallest control system sample time value. This can be done by typing 'Ts = 20e-6' in the workspace in the case of this example. See also dc7_example_simplified model.

WORKING:

• A four-quadrant chopper is a chopper which can operate in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In the first quadrant, a Class-E chopper acts as a Step-down chopper whereas in the second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper. This article describes the working principle and operation of Class-E chopper with the help of circuit diagram.

Working Principle / Operation of Class-E Chopper:

• The circuit of a four-quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

• In the above circuit diagram, the choppers are numbered CH1 to CH4. For the first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will discuss its operation separately for each quadrant.

First Quadrant Operation:

• For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that v_o = v_s. It may be noted that the load current flows from source to load as shown by the direction of i_o.
• When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remain positive.
• Thus, both the output voltage v_s and load current i_o are positive and hence, the operation of chopper is in first quadrant. It may be noted that Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

• To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the on period of CH2.
• When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is the second quadrant operation of chopper. Since the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read Step-up chopper for detailed analysis and better understanding.
• For the second quadrant operation, load must contain emf E as shown in the circuit diagram. In the second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

• To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what is shown in the circuit diagram. If the direction is opposite to what is shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

• For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, v_o is assumed negative. Let us now see what the status of load current is. It may be seen that i_o is flowing in the direction opposite to shown in the circuit diagram and hence negative.
• Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, v_o and i_o are both negative. Hence, the chopper operates in the third quadrant.

Fourth Quadrant Operation:

• To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.
• When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in the fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.
• The operation of a four-quadrant chopper or Class-E chopper is summarized in the figure below.

SIMULINK MODEL:

RESULTS:

COMPARISON BETWEEN H BRIDGE AND FOUR QUADRANT DC CHOPPER:

• A four-quadrant chopper is a chopper which can operate in all the four quadrants. The power can flow either from source to load or load to source in this chopper whereas a H-Bridge circuit can operate only in two modes either in forward motoring or reverse motoring.
• The variation in the armature current is much higher in four-quadrant chopper as compared to a H-Bridge.
• An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load but in case of a four-quadrant chopper the polarity of the voltage applied to a load is not changed but the polarity of the load EMF is reversed.

OBJECTIVE - 2:

TWO - QUADRANT CHOPPER:

• Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to Vs. The direction of the load current i₀ will be reversed. The current i0 will be flowing towards the source and it will be positive regardless of whether the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in the first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.
• The average voltage will always be positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation i.e. from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring this type of chopper configuration is used.

WORKING:

• Sometimes a chopper may be required to provide a two-quadrant operation by retaining the current direction in both motoring and braking modes. Such a Two Quadrant Chopper is shown above. The chopper permits a change in the polarity of terminal voltage keeping the direction of current constant. S₁ and S₂ are the choppers working as electronic switches and D₁ and D₂ are the feedback diodes. Assuming continuous conduction, both S₁ and S₂ operate simultaneously. Sometimes a chopper may be required to provide a two-quadrant operation by retaining the current direction in both motoring and braking modes. The chopper permits a change in the polarity of terminal voltage keeping the direction of current constant. S₁ and S₂ are the choppers working as electronic switches and D₁ and D₂ are the feedback diodes. Assuming continuous conduction, both S₁ and S₂ operate simultaneously.
• When they are ON, the load current I_d increases, with the rate of rise depending upon the back emf and torque of the machine. When they are OFF, the load current is fed to the supply through the diodes D₁ and D₂. The current decreases at this stage. It is clear that the load voltage is effectively negative when the diodes conduct. The average value of the output voltage of the chopper V_da is zero for a = 0.5, the output voltage is an ac waveform with average voltage equal to zero. For a > 0.5 the average value of the dc voltage is negative. The chopper works in regeneration mode. The power flows from the load to the supply and the dc machine operates as a generator. For control ratios a c 0.5, the operation is similar to the inversion of a line comutated converter (α > 90°). For a > 0.5 the chopper has normal operation. The power flows from the supply to the load. This operation of the chopper is similar to rectification of a line commutated converter (α < 90°). For, a = 0.5 the load voltage is zero.

SIMULINK MDEL:

RESULTS:

OBJECTIVE - 3:

BLDC MOTOR:

• A brushless DC motor (also known as a BLDC motor or BL motor) is an electronically commuted DC Motor which does not have brushes. The controller provides pulses of current to the motor windings which control the speed and torque of the synchronous motor.
• These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.

CONSTRUCTION OF BRUSHLESS DC MOTOR:

• In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.
• The main design difference between brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

• Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.
• The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “In runner” style.

  • Out runner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

  • In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

WORKING PRINCIPLE OF BRUSHLESS DC MOTOR:

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor is placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the case of the BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching generates an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
• Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles aligns with stator poles causing motor to rotate.
• Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.

• Here, one might get a question that how we know which stator coil should be energized and when to do. This is because the motor continuous rotation depends on the switching sequence around the coils. As discussed above, Hall sensors give shaft position feedback to the electronic controller unit.
• Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low- and High-level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

ADVANTAGES OF BRUSHLESS DC MOTOR:

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Low noise due to absence of brushes

LIMITATIONS OF BRUSHLESS DC MOTOR:

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need for additional sensors

APPLICATIONS OF BRUSHLESS DC MOTOR:

Brushless DC motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc.

• Computer hard drives and DVD/CD players
• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt driven systems
• Washing machines, compressors and dryers
• Fans, pumps and blowers.

CONCLUSION:

• From the above assignment we learnt about the H-Bridge circuit, Four-Quadrant DC Chopper and Two-Quadrant DC chopper and their working principle.
• We learnt about the Speed control of a DC motor using BJT H-Bridge circuit.
• We learnt about the Four Quadrant Chopper DC drive and compared the two of them.
• We analyzed and commented about the armature current shoot-up in case of the H-Bridge circuit.
• We developed a Two-Quadrant DC Chopper in Simulink and observed the results.
• We learnt about the BLDC motor.
• We learnt about the construction, working principle, advantages, limitations and applications of a BLDC motor.